Flow meters are used to measure the mass flow rate, density, and other characteristics of flowing materials. The flowing material may comprise a liquid, gas, solids suspended in liquids or gas, or any combination thereof. Vibrating conduit sensors, such as Coriolis mass flow meters and vibrating densitometers typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, lateral, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Density of the flow material can be determined from a frequency of a vibrational response of the flow meter. Two such transducers (or pick-off sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits and are typically located at positions upstream and downstream of the actuator. The two pick-off sensors are generally connected to electronic instrumentation by cabling, such as by two independent pairs of wires. The instrumentation receives signals from the two pick-off sensors and processes the signals in order to derive flow measurements.
In operation, the flow tubes are driven out of phase with respect to one another. The drive force is generated by an electro mechanical driver which generates out of phase vibrations of the flow tubes at their natural resonance frequency. For discussion purposes, the flow tubes may be said to be driven in a vertical plane by the driver. These vertical vibrations are relatively large since they are at the first out of phase bending mode of the flow tubes and they are driven at their resonance frequency.
The Coriolis deflections of the vibrating flow tubes with material flow also occur in the same vertical plane as the drive vibrations. The Coriolis deflections occur at the drive frequency but the tube deflections have the shape of a bending mode with a higher frequency. Therefore, the amplitude of the Coriolis deflections is considerably less than the amplitude of the flow tube drive frequency vibrations. Even though the amplitude of the Coriolis response is relatively small, it is the Coriolis response that generates the pick-off output signals that are processed by meter electronics to generate the desired mass flow rate and other information pertaining to the flowing material. Many Coriolis flow meters are capable of obtaining an output error of about 0.15% or less. However, in order to achieve this accuracy, noise and unwanted signals must be minimized.
In the operation of a Coriolis flow meter, the signals induced in the pick-offs comprise not only the desired small amplitude Coriolis response signals, but also comprise unwanted signals that are applied to the processing circuitry along with the desired Coriolis response signals. These unwanted signals impair the ability of the processing circuitry to generate accurate output signals.
The unwanted pick-off signals may be caused by ambient noise from the surrounding environment. Ambient noise may be due to near-by machinery and the like. It may also be caused by vibrations in the pipeline to which the Coriolis flow meter is connected. Ambient noise can be overcome by proper mounting of the flow meter to isolate it from the external vibrations. The noise from connected pipeline vibrations can be overcome by appropriate isolation of the flow meter from the pipeline.
Another source for unwanted signals is unwanted vibrations in the flow meter. These unwanted vibrations are more difficult to overcome and can be minimized, but generally cannot be eliminated, by improving the flow meter design.
Most vibrating flow meters have various mode shapes that result from driving the flow meter at its resonant frequency. A typical flow meter can have vibrational modes that are characterized by their shapes as follows:
In-phase bend (IPB)
In-phase lateral (IPL)
Out-of phase bend (Drive)
Out-of phase lateral (OPL)
The out of phase bend is generally the desired drive mode while the rest are typically unwanted modes. The above mentioned modes are inherent in most vibrating flow meters, including Coriolis flow meters. The frequency of these modes generally changes with the density of the flowing material. When a mode changes frequency, there is a potential for interaction between neighboring modes that may cause the flow meter to become unstable and produce incorrect output data. As mentioned above, the mode that is desired and used to generate the desired output information of the flow meter is the out of phase bend drive mode. It is this mode that generates the Coriolis forces. The resulting Coriolis response is detected by the pick-offs, which generate the signals that are used to provide the flow meter output information.
The in phase lateral and out of phase lateral vibrations can create a problem when processing the signals received by the pick-offs representing the Coriolis forces. The lateral mode vibrations are typically offset from the drive plane. The lateral mode vibrations are generally substantially perpendicular to the drive mode vibrations. The lateral plane is substantially transverse to the applied oscillation.
One method of minimizing the adverse effects of the two different lateral frequencies is to increase the separation between the drive mode frequency and the unwanted lateral frequencies. If these undesired lateral mode signals have excessive amplitude and/or are close to the frequency of the Coriolis response signal, the electronic processing circuitry may be unable to process the Coriolis signal to generate output information having the desired accuracy.
It may be appreciated from the above that it is a problem in the design and operation of Coriolis flow meters to minimize the adverse impact of signals generated by undesired modes of vibration so that the processing of the Coriolis response signal and the output accuracy of the output signal of the flow meter is not compromised.
There have been a number of prior art approaches that have attempted to increase the separation of the drive mode frequency and the lateral mode frequency. One such approach is provided in U.S. Pat. No. 6,314,820 assigned to the present applicant. The '820 patent incorporates lateral mode stabilizers that slide over the flow tube and include extensions that extend inward to stiffen the lateral portion of the flow tube to raise the lateral vibration frequency. The stabilizers are held using a balance bar.
Although the method disclosed in the '820 patent provides adequate results, it requires excessive number of parts in addition to the balance bar. In addition, although the lateral mode stabilizers may be implemented in a curved flow tube design, they are more applicable to straight tube designs.
Another prior art approach is disclosed in U.S. Pat. No. 5,115,683, which uses a brace attached to the flow tube near the driver on one end and attached to a base on the other end. The brace is flexible to allow motion of the flow tube due to the Coriolis reaction but limits the ability of the flow tube to displace laterally. Again, the '683 requires excessive number of parts that are subject to damage.
Another prior art approach is disclosed in U.S. Pat. No. 6,354,154, assigned to the present applicant, which uses a balance bar with side ribs that inhibit the undesired lateral vibrations to raise the frequency of the lateral vibrations. U.S. Pat. No. 6,598,489 uses a similar idea as the '154 patent but shapes the ribs to raise the resonant frequency of the drive mode versus the lateral mode. A limitation of both the '154 patent and the '489 patent is the requirement of a balance bar. Because balance bars are typically not implemented in dual flow tube flow meters, this approach has limited applicability.
Another prior art approach is disclosed in U.S. Pat. Nos. 7,275,449 and 4,781,069 both of which disclose the use of plates or braces that connect the two flow tubes together in a way that increases the lateral mode frequency in order to separate it from the drive mode. A problem with this approach is that because the plates connect two separate flow tubes together, the drive mode may also be adversely affected. This may be especially true for low flow rate applications.
Therefore, there exists a need in the art for a flow meter design capable of separating at least two modes of vibration. Furthermore, there exists a need to separate at least two modes of vibration without requiring excessive parts. The present invention solves this and other problems and an advance in the art is achieved.
Aspects
According to an aspect of the invention, a flow meter including one or more flow tubes and a driver adapted to vibrate the one or more flow tubes at a drive frequency, the flow tube comprises:                a gusset coupled to and extending along the flow tube such that a frequency separation between the drive frequency and at least a second vibration frequency is increased.        
Preferably, the gusset extends along a portion of the flow tube.
Preferably, the gusset extends along substantially the entire flow tube.
Preferably, the gusset couples two or more portions of the flow tube together.
Preferably, the at least second vibration frequency comprises a lateral vibration mode.
Preferably, the gusset is coupled to the flow tube such that a portion of the flow tube is stiffened.
Preferably, the gusset is adapted to raise a frequency of a lateral vibration mode.
Preferably, the gusset is formed as an integral part of the flow tube.
According to another aspect of the invention, a flow meter including one or more flow tubes and a driver adapted to vibrate the one or more flow tubes at a drive frequency, the flow tube comprises:                a gusset coupled to and extending along the flow tube such that a portion of the flow tube is stiffened.        
Preferably, the gusset extends along a portion of the flow tube.
Preferably, the gusset extends along substantially the entire flow tube.
Preferably, the gusset couples two or more portions of the flow tube together.
Preferably, the gusset is adapted to increase a frequency separation between two or more vibration modes.
Preferably, the gusset is adapted to increase a separation between a frequency of the drive vibration and a frequency of a lateral vibration.
Preferably, the gusset is adapted to raise a frequency of a lateral vibration.
Preferably, the gusset comprises an integral portion of the flow tube.
According to an aspect of the invention, a method for increasing a separation between two or more vibration frequencies of a vibrating flow meter including one or more flow tubes and a driver configured to vibrate the one or more flow tubes at a drive frequency in a drive plane, the method comprises the step of:                coupling a gusset to the flow tube, such that the separation between two or more vibration frequencies is increased.        
Preferably, the step of coupling the gusset to the flow tube comprises extending the gusset along a portion of the flow tube.
Preferably, the step of coupling the gusset to the flow tube comprises extending the gusset along substantially the entire length of the flow tube.
Preferably, the step of coupling the gusset to the flow tube comprises coupling two or more portions of the flow tube together.
Preferably, the two or more vibration frequencies comprises the drive frequency and a lateral vibration frequency.
Preferably, the step of coupling the gusset to the flow tube comprises coupling the gusset to two or more portions of the flow tube such that the frequency of a lateral vibration mode is increased.
Preferably, the step of coupling the gusset to the flow tube comprises coupling the gusset to two or more portions of the flow tube such that a portion of the flow tube is stiffened.